Kinetics of Oxidation of Atropine by Alkaline KMnO4 in Aqueous Solutions

Atropine sulfate monohydrate (ASM, Fig. 1) is pharmaceutical formulation that is one of the World Health Organization’s list of essential medicines. Atropine itself is a naturally occurring tropane alkaloid with chemical structure similar to that of cocaine. Plants of the family Solanaceae that includes Atropa belladonna, Datura stramonium and Mandragora officinarum are the major natural sources of the alkaloid. Atropine serves as a drug with a wide variety of effects. It is a competitive antagonist of the muscarinic acetylcholine receptors, hence its major effects are on the parasympathetic nervous system. As an anticholinergic drug (parasympatholytic) it is considered a nonselective muscarinic acetylcholinergic antagonist. Consequently, its main medical uses are based on these potent biological properties. Since it blocks the action of the vagus nerve, atropine is used in resuscitation procedures. Additionally, atropine mixed with pralidoxime chloride (2-PAM chloride) is used as an antidote for poisoning by organophosphate insecticides and nerve gases, such as Tabun (GA), Sarin (GB), Soman (GD) and VX. Atropine has also several medical applications that include ophthalmic uses as therapeutic mydriatic and in slowing the progression of myopia in children . As much as it is crucially important to understand this kind of physiological activity, atropine interactions with common chemical reagents are still lacking. Oxidation of atropine by Kinetics of Oxidation of Atropine by Alkaline KMnO4 in Aqueous Solutions


INTRODUCTION
Atropine sulfate monohydrate (ASM, Fig. 1) is pharmaceutical formulation that is one of the World Health Organization's list of essential medicines 1 . Atropine itself is a naturally occurring tropane alkaloid with chemical structure similar to that of cocaine. Plants of the family Solanaceae that includes Atropa belladonna, Datura stramonium and Mandragora officinarum are the major natural sources of the alkaloid. Atropine serves as a drug with a wide variety of effects 2 . It is a competitive antagonist of the muscarinic acetylcholine receptors, hence its major effects are on the parasympathetic nervous system. As an anticholinergic drug (parasympatholytic) it is considered a nonselective muscarinic acetylcholinergic antagonist. Consequently, its main medical uses are based on these potent biological properties. Since it blocks the action of the vagus nerve, atropine is used in resuscitation procedures 3 . Additionally, atropine mixed with pralidoxime chloride (2-PAM chloride) is used as an antidote for poisoning by organophosphate insecticides and nerve gases, such as Tabun (GA), Sarin (GB), Soman (GD) and VX 4 . Atropine has also several medical applications that include ophthalmic uses as therapeutic mydriatic and in slowing the progression of myopia in children 5,6 .
As much as it is crucially important to understand this kind of physiological activity 7 , atropine interactions with common chemical reagents are still lacking. Oxidation of atropine by Asian Journal of Chemistry; Vol. 27, No. 10 (2015), 3877 alkaline copper(III) periodate complex has been recently reported 8,9 . Organic oxidation protocols have commonly employed transition metal complexes in their higher oxidation states that can be stabilized by appropriate polydentate ligands 10,11 . Atropine oxidation by typical strong oxidizing reagents including potassium dichromate and potassium permanganate in appropriate pH media have never been reported to the best of our knowledge. Understanding the behaviour of atropine when treated with powerful oxidizing agents in aqueous media might shed some light into its interaction with oxidative enzymes such as FAD and NADP 12 .
Kinetics of oxidation of organic substrates by alkaline KMnO4 has been reported in literature and demonstrated to be a powerful mechanism elucidating tool. Examples include oxidation of DL-aspartic acid and sugars 13,14 .
It is assumed that, under the reaction conditions and stoichio-metries used herein, oxidation of the aliphatic alcohol of atropine to a carboxyl group is the predominant reaction 15 (Scheme-I). Permanganate ion attains its optimum stability in neutral to slightly alkaline media 16 . However, in strongly alkaline media, it disproportionates to manganese(V) (hypomanganate) or manganese(VI) (manganate). Hence, for oxidation reactions under these conditions it will be difficult to determine whether a one or two electron process is operating 12 . To sort out the exact reaction of basic KMnO4 with atropine a set of preliminary experiments were done.
In the present work, as an extension of our previous studies 17-20 on both kinetics and biological application to widen our knowledge about the oxidation of biologically important substances, we report the kinetics of oxidation of atropine by alkaline KMnO4. Kinetic results at various temperatures are presented and a proposed mechanism for the oxidation process is given and discussed.

EXPERIMENTAL
Alkaline hydrolysis: Atropine sulfate monohydrate (0.10 M) was treated with KOH (1.0 M) solution for 25 min (optimum run time frame for reaction monitoring, arbitrary set). Hydrolysis products were then isolated, purified and characterized by NMR and IR spectroscopy. Results indicated that hydrolysis did not proceed to any measurable extent. However, the addition of MnO2 greatly enhanced the hydrolysis.
Alkaline KMnO4 disproportionation: A blank run of mixing all ingredients [KMNO4 (0.001 M), KOH 0.10 M)] except atropine sulfate monohydrate has shown that the change in absorption by KMnO4 is negligible throughout the measuring period.
Alkaline KMnO4 oxidation: Atropine sulfate monohydrate solution (0.01 M) (about 10 folds in excess) was treated with the alkaline KMnO4 (0.001 M) solution for 25 min. Products were isolated, purified and thoroughly characterized. The products were excess atropine, tropine, tropic acid and phenylmalonic acid (Scheme-I). Addition of mercuric chloride or acrylonitrile monomer to the reaction system, directly after the appearance of the green colour of MnO4 2-, induced a white precipitate formation and polymerization, respectively. Consequently, a radical formation throughout the course of the reaction mechanism cannot be ruled out 21,22 . The reaction was quenched and the green colour persists.
These results implicated that the major change in the oxidant concentration (absorbance at its maximum wavelength) is attributed to the oxidation of atropine primary alcohol. Hence, kinetic measurements were designed to follow up this reaction. It is aimed to investigate the mechanism by which the reaction is taking place. Kinetics measurements: Freshly prepared aqueous solutions of desired concentrations of atropine sulfate monohydrate and KMnO4 were used in the kinetic study. In a strongly basic solution (0.1 M), atropine sulfate monohydrate is converted to atropine. Measurements were carried out using a Diode Array Spectrophotometer model 8453E from HP Agilent Technologies. Reactions were monitored by following the change in absorbance of (MnO4 2-) reaction mixture with time at a predetermined wavelength. The wavelength was determined by recording the absorption spectra for KMnO4 alone and for its mixture with atropine sulfate monohydrate after completion of the reaction. The wavelength of maximum absorbance difference (λmax), preferably in visible region of the spectra, between the absorption of KMnO4 and the reaction mixture was selected. A λ scan for reactants and products at various time intervals is shown in Fig. 2, from which λmax = 440 nm was selected. At this wavelength absorbance is mainly due to MnO4 2and absorbance due to all other Mn species is minimal. Also, it should be noted that none of the organic compounds, reactants or products, absorb at this wavelength. All reactions were studied under pseudo-first order conditions, at which the concentrations of atropine sulfate monohydrate [10 -2 to 10 -1 mol dm -3 ] were at least an order of magnitude larger than those of KMnO 4 [10 -4 to 10 -2 mol dm -3 ]. The pH of the reaction mixture was maintained constant at 13.0 ± 0.1 and its ionic strength was fixed using NaClO 4 . Rates were measured at various temperatures and for each measurement it was maintained constant within ± 0.1 °C.

RESULTS AND DISCUSSION
The main reactions of oxidation of atropine by KMnO4 in basic medium are shown in Scheme-I. Since kinetics measurements were run under pseudo first order condition, in which the concentration of atropine is an order of magnitude larger than that for KMnO4, then, the first stage of the process which involves the oxidation of atropine by alkaline KMnO4 is given by the following equation: An excess of atropine remains. The excess atropine is expected to undergo partial hydrolysis due to the presence of MnO 2 as byproduct which acts as an auto-catalyst to give tropine and tropic acid and the oxidized product in stage I is also expected to undergo an alkaline hydrolysis to yield tropine and phenylmalonic acid (Scheme-I). The rate of oxidation of atropine is given by: where k obs is the observed rate for reaction is given by: where k is the rate constant for eqn. 1. For a first-order dependence of reaction rate on [KMnO 4 ], experimental absorbance-time data pairs were fitted to the exponential function: where At is the absorbance of the reaction mixture which is mainly due to MnO4 2at a given time (t) through the reaction, A0 is its initial absorbance (t = 0) and A∞ is the absorbance of the mixture at the end of the reaction, that is when the absorbance no longer changes with time (t = ∞).
Experimental results showed that a plot of ln [(A∞ -At)/ (A∞ -A0)] vs. time gives a straight line. Its slope represents the rate of appearance of MnO4 2-, which equals the rate of disappearance of MnO4 -(as shown in the suggested mechanism below). The value of kobs (s -1 ) was obtained from the slope, according to eqn. 4. Using eqn. 3, a plot of ln kobs vs. ln [ATN] a is a straight line that gives the reaction rate constant, k, in units of dm 3 mol -1 s -1 (intercept) and the order of the reaction (a) with respect to [ATN] (slope). At specific concentrations of substrate and oxidant several trials were carried out for each measurement and the average value of the observed rate is reported. Two observed rates were found (kobs1 and kobs2) and hence, two different reaction rate constants are obtained. Experimental errors in value of rate constant are estimated to be ± 10 %.
The kinetics results for the oxidation of atropine by KMnO4 are shown in Table-1   Based on the obtained kinetics results, we propose the following mechanism for the oxidation process of atropine.
Step 1 Step 3 Step 4 Step 6 MnO 4 2− disproportionates to MnO 2 (10) where C and D are complexes, I and II are intermediates.
In a strongly basic solution, MnO 4 will be in the form of [MnO 4 .OH] 2-. Experimentally, the reaction rate was followed by monitoring the change in absorbance of the reaction mixture with time at λ = 440 nm. At this wavelength, the absorbance (A t ) is mainly due to the formation of MnO 4 2- (Fig. 2) and it is increasing with time. Due to regeneration of MnO 4 from MnO 4 2-(eqn. 7), then the rate of formation of MnO 4 2is approximately equal to the rate of disappearance of MnO 4 -.
Oxidation of primary alcohols under strongly alkaline KMnO 4 is known to occur in two stages 23 . For the first stage, an aldehyde intermediate is well-established. Further oxidation of the aldehyde to the final product (carboxylic acid) proceeds at a faster rate compared to the first stage.  25 is reached, the reaction will be controlled by step 5 (eqn. 9) that is faster than the first limiting step 3. Finally oxidation of intermediate II proceeds cleanly to produce the final products. .00 .00 .00 .00 .00 .00 ln (A -A )/ The change in absorbance with time represents the rate of the reaction according to eqn. 2. From a kinetic point of view, steps 2 and step 5 will be the rate determining steps and steps 1, 3, 4, 6 are fast. Experimentally, two rates were observed. The first reaction rate (rate 1) is due to step 2, consequently, eqn. 1 above can be rewritten as: Rate 1 = k 2 [C] and from eqn. Here, k 2 K 1 is a constant quantity which represents the first rate of the reaction and will be represented as k rxn1 .
A strong reason why a λ max at 440 nm, which is specific for MnO 4 2-, was selected is due to the formation of MnO 4 2in both of the slow steps 2 and 5. The disappearance of the distinct permanganate absorbance band located in the range of wavelengths 490-590 nm and the formation of bands with maxima at 347, 439 and 606 nm (Fig. 2) are characteristic of manganate MnO 4 2-, supports this observation. Disproportionation of manganate to form permanganate and manganese dioxide is then follows 25 . Possible structures of the intermediates and complexes involved are shown in Fig. 8. It is worth mentioning that the proposed structures of these intermediates are not conclusive but highly possible 26   The above proposed mechanism is supported by the observation of initial colour change of permanganate from violet to green (manganate). The radical mechanism was invoked due to the precipitation observed when mercuric chloride was added to the system 28 . When HgCl2 was added after the appearance of the green colour, the colour persists. The reaction was quenched at this stage.
Since the observed overall rate was obtained to be first order with respect to permanganate, eqns. 11 and 12 above will have the following expression The rates constant krxn1 and krxn2 were obtained from plots of ln kobs1 vs. ln [ATN] and ln kobs2 vs. ln [ATN], respectively.
Effect of temperature on rate constant was studied. Results are shown in Table-2. Each value in the table is the average of several runs. Experimental errors in the rate constant are estimated to be ± 10 %. As expected, results show that the rate constant (krxn2) for step 5 is greater than that for step 2 (krxn1). The activation energy (Ea) for both steps 2 and 5, were calculated using Arrhenius equation. From plots of ln k vs. (1/T) for both steps (Figs. 9 and 10), Ea's of 25.64 and 20.69 KJ for steps 2 and 5, respectively. Apparently; the thermodynamics is in  agreement and proportional to the related rate constants determined for these steps.

Conclusion
Reaction and kinetics of the oxidation of the biologically important atropine by alkaline KMnO4 was studied. Experimental observations show that the reaction proceeds via two stages to give the final products: tropine and phenylmalonic acid. Two rate determining steps with different rates were proposed, which were in agreement with obtained kinetic result. A proposed mechanism for the process is presented. The mechanism suggests the formation of two intermediates in two stages of the reaction with different rates and activation energies.